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1. Field of the Invention
The present invention relates to multi-channel audio reproduction, and more particularly, to the control of audio images in a listening space.
2. Related Art
Reproducing the sound we hear in life is split into two related challenges, namely fidelity and position. Over the past 100 years the fidelity of reproduced sound has improved considerably but only recently with the introduction of multi-channel digital audio storage devices such as DVD and digital hard drive recorders has it been possible to play multiple channels of audio in a wide variety of venues.
The number of channels of recorded audio is controlled by the recording format but unfortunately as the work of Haas and others show each point source in a space is identified as such by the human brain and so an immersive realistic sound is not created. The precedence effect, in which the human brain localizes to the first sound heard of a sample, forces the image to the audio source closest to the listener. The area equidistant from the loudspeakers is often called the sweet spot and is regarded as the optimum listening position but is unfortunately small, often limited to one or two listeners.
To create the illusion of movement of sound in a listening space or venue, traditionally engineers have used pan pots to gradually lower the sound level in one loudspeaker and simultaneously gradually raise it in another in a process known as panning. However, outside the sweet spot this does not create a realistic illusion of movement due to the precedence effect.
The following references have made contributions in the technology. U.S. Pat. No. 6,663,648 to Bauck, which proposes solutions to the small sweet spot by physical modifications to loudspeakers to move the high frequency drivers closer to the center. U.S. Pat. No. 6,307,941 to Tanner, Jr. et al. proposes and sweet spot enhancing solution for static situations using time delay processors and filtering techniques. Also, in Australian Patent Application No. PQ9424, McGrath et al. provides a loudspeaker system for audio-visual production with delays for loudspeakers is described.
Reproduction of multi-channel sound in the cinema is well established in the art, with noted format standards including Dolby.RTM,™ DTS™ and ProLogic™ formats, to name a few. The most common standard for Surround Sound is 5.1 whereby the audio signals are stored as left, center, right, surround left and surround right. There are also adaptations of the foregoing to include one rear channel in 6.1 and two rear channels in 7.1 formats. In these systems, the “0.1” channel is defined as low frequency channels, used for certain special effects, and is in mono as such frequencies are not processed by the human brain with any significant positional information. The 5.1 audio format is often misunderstood as a listening environment, whereas in actually it is a recording and storage medium. Also, suggestions have been put forward for a 10.2 system, which effectively doubles the number of channels.
Helmut Haas, in a doctoral dissertation to the University of Gottingen, Germany as “Uber den Einfluss eines Einfachechos auf die Horsamkeit von Sprache;” discloses what has become called the “Hass effect” or “precedence effect,” notably that in the frequency range 500 Hz to 2000 Hz, the time differences between identical sounds arriving at human ears will be dominant in deciding the origin of that sound. In summary, Haas defines the precedence effect to mean that when multiple identical sounds arrive at a listener, but at different times, the position information of the first sound takes precedence over the later arrivals of the same sound. This effect occurs up to the onset of echo perception, at approximately 40 milliseconds.
Existing solutions generally require a direct relationship between the number of recorded channels in the media and the number of speakers. In a multi-seat room such as a cinema, the listening experience is different for each seat because of the proximity of the loudspeakers and the precedence effect described above. Referring to FIG. 13, a listener 900 respectively hears feed signal 910 from Ls speaker 902, 912 from L speaker 302, 914 from C speaker 908, 916 from R speaker 302, 920 from Rs speaker 906, 1308 from Rb speaker 1304, and 1302 from Lb speaker 1302. FIG. 14 illustrates the same signals in relation to a listener in a movie theatre. From the seating position of listener 900 in a small space in FIG. 13, the same effects are observed as in the movie theatre space of FIG. 14, except that the loudspeakers in FIG. 14 are further apart. Noticeable gaps in the sound image between the loudspeakers shown have been demonstrated to make the effect less realistic.
Adding more loudspeakers 902, 906 in parallel, as shown in FIG. 15, provides greater coverage, but creates multiple sound sources. The multiple sound sources cause a confusing sound field due to multiple sound arrivals from the different sound paths having the same program material. The system depicted in FIG. 14 is in common use in cinemas at present.
As shown in FIG. 16, electronic delays 1602, 1604, 1606, 1608 can be inserted into the loudspeaker feeds. However, the illustrated structure only works for a small area in the center of the room as shown. For a listener located in the left for the room, for example, the delay patterns vary from the ideal situation of FIG. 16, and the image may be lost.
What is needed is the ability to overcome the foregoing problems to provide a realistic listening experience for surround sound, to widen the sweet spot listening area to encompass the majority of the audience in public auditorium, to add realistically moving sound effects, and to enable providing of recording and delivery formats independent of each other.